Satellite communications have many practical applications for a device that operates in two or more frequency bands. Many devices have been employed to realize dual band frequency and/or multi-band frequency operation including frequency selective sub-reflectors, broad band feed networks, and coaxial orthomode transducers.
Typically, in the past, coaxial OMTs have been designed with circular inner conductors and circular outer conductors. To separate the lower frequency band polarizations, these designs utilized axially co-located ports, probes, or axially separate ports with tuning screws. Axially co-located ports may be the best technical solution but are very expensive to manufacture. Probe or tuned solutions may be less expensive to realize, however, tend to have poor port to port isolation and return loss. Additionally, these solutions also couple to HOM's and/or the undesired TEM mode.
A disadvantage of the undesired TEM mode includes the generation of cross-polarized electromagnetic signal components as a result of slight asymmetries in the conductor(s). Cross-polarization components may negatively effect efficient electromagnetic signal propagation. Cross-polarization components may be created by a rotation of the principal plane of the electromagnetic signals propagating through the conductor from the desired plane to a plane between the desired plane and the plane of the cross-polarization components. This rotation of the polarization plane results in power transfer to the cross polarized component, thus causing a decrease in efficiency.
A disadvantage of the HOM includes the propagation of electromagnetic signals at velocities other than the desired velocity for efficient electromagnetic signal transmission. High HOM coupling, therefore, also results in a decrease in efficiency.